Organofunctional silane compounds have been employed in the treatment of a variety of surfaces, such as metal oxides, silicates, particulate siliceous fibers and pigments, and fibers such as glass fibers, steel fibers and aluminum fibers. (Metal surfaces are regarded as oxide surfaces because they are oxidized even though their subsurfaces are not.) Some organofunctional silane treatments involve coating such a surface with an organic, an aqueous-organic or aqueous solution of the silane either alone or in conjunction with other chemicals.
These treatments enhance bonding between the inorganic oxide and resinous media. Consequently, the silanes have utility as components in primers in the application of coatings, adhesives and sealants to inorganic oxide surfaces and as a filler pretreatment to improve the strength and structural integrity of filled resin composites such as those incorporating glass fibers. Such organofunctional hydrolyzable silanes are termed "Coupling Agents" or "Adhesion Promoters".
The efficacy of the organofunctional silane is dependent in large part on its ability to react chemically or bond with the resinous media. During the curing reactions of thermosetting resins, the reactivity of the organofunctional group needs to match the reactivity of the thermosetting resin or the curing agent. It must become part of the organic polymer that makes up the resinous media. In fabricating thermoplastic articles, the organofunctional silane also needs to become part of the resinous media, either by reacting with organofunctional groups on the polymers or grafting onto the polymer backbone.
The organofunctional silane need not react with all of the resinous media to achieve the enhancement in structural integrity of the article incorporating it. Often, the enhancements can be achieved if the silane reacts with only a part of the resinous media or with additives that are compatible with the resinous media. For example, virgin polypropylene destined for glass fiber reinforcement typically is compounded with maleated polypropylene (a.k.a. coupled polypropylene) as taught in U.S. Pat. No. 5,300,547. Maleated polypropylene refers to a graft or co-polymer of propylene and unsaturated organic acids or their anhydrides. For example, maleic anhydride commonly is reacted with propylene to make maleic anhydride-propylene co-polymers. The two polypropylenes, virgin and maleated, along with glass fibers are co-fed into a compounding extruder and pelleted. Glass fibers for this process are manufactured with surface treatments (sizing) to include silanes chemically compatible with the maleic anhydride. An amino silane, such as SILQUEST.RTM. A-1100.TM. 3-aminopropyltriethoxysilane, would be an example of a maleic anhydride compatible silane. The silane on the glass fibers and the maleated polypropylene form chemical bonds during compounding (extrusion). The silane does not react with the virgin polypropylene. This chemical bonding of the glass fiber to the maleated polypropylene is responsible for the performance advantages of the glass fiber reinforced polypropylene. One disadvantage of using maleated polypropylene is its cost relative to that of uncoupled or virgin polypropylene. Another disadvantage of using maleated polypropylene is that the maximum enhancements in structural integrity and strength are not achieved. The maleated polypropylene mixing or dispersion into the virgin polypropylene may not be uniform. Its ability to react chemically with the organofunctional group of the silane on the inorganic surface is inhibited by the presence of the virgin polyproplyene. The maleic functional groups of the polypropylene that do not react with the silane can contribute undesirable properties to the composite, such as making the resinous media more hydrophilic and, therefore, less resistant to degradation due to moisture. Achieving increased performance with glass fiber reinforced virgin polypropylene, by providing a glass fiber reactive toward the uncoupled (virgin) polypropylene, would be useful.
Moreover, other organic resins (e.g., nylon) used in conjunction with inorganic materials often have insufficient or non-reactive functional groups. New silanes that are effective at bonding with these materials are required (needed) to enhance the organic resin's properties when used with inorganic materials. Imidosilanes, when used alone or in combination with free radical generators are unique in their ability to couple with or bond to various resinous media.
Imidosilanes are known, see e.g., U.S. Pat. Nos. 3,755,354 and 3,249,461 and EP Patent No.0 50861, but these imidosilanes are problematic in that the presence of water is inherent in the method of manufacture of said silanes. Water causes hydrolysis of the silane to form siloxanes and turns the silanes from a liquid to a paste, which is unsuitable for uses in coating or sizing formulations that require the silane to be soluble and flowable over the surface of the inorganic surface. See, e.g., Example 1 of U.S. Pat. No. 3,755,354. Other synthetic methods result in the presence of chloride, which degrades the composite matrix.